Tools used in connection with drilling of oil and gas wells are subject to considerable abrasion and wear during use. For example, mud-lubricated radial bearings used as drill bits, rock mills, mud motors, are subject to highly abrasive particles found in drilling fluids and frequently require replacement. Metal carbides, particularly tungsten carbide (WC), are used to form a bearing or wear surface for downhole tools because of their desirable properties of hardness, toughness and wear resistance. There are a number of different methods for applying tungsten carbide to a substrate or support to form a wear surface for a bearing.
Conforma Clad®, a brazed tungsten carbide cladding, sold by Kennametal, is one example of a tungsten carbide wear surface that can be used for bearings. Examples of commercially available products with Conforma Clad® wear surfaces include radial bearings used in downhole mud motors. These tungsten carbide clad surfaces are fabricated by overlaying a surface of an object to be clad to form a wear surface with a cloth containing tungsten carbide powder, mixed with a binder, and then laying on top of it another cloth containing a braze alloy before subjecting the part to heating to melt the binder phase and the braze.
Another method, described in U.S. Pat. No. 4,719,076, uses a blend of macro-crystalline tungsten carbide powder and cemented tungsten carbide cobalt chips to create a hard wear surface on a radial bearing. The mixture described in U.S. Pat No. 4,719,076 is comprised of sixty percent by weight of 80 mesh macro-crystalline tungsten carbide, commercially available as the Kennametal product called P-90, and forty percent by weight of cemented tungsten carbide cobalt crushed chips with a mesh size of 10/18. The percentage of weight of tungsten carbide cobalt chips is from forty to eighty percent by weight. To create a bearing surface, a steel blank is surrounded by a graphite mold and the blended mixture of macro-crystalline tungsten carbide and cemented tungsten carbide chips is loaded into a cavity created between the steel blank and graphite mold. After the mold contents are vibrated to achieve maximum density of the blended mixture tungsten powder, copper based infiltrant is then placed in a funnel shaped ring formed around the top of the mold. The mold is then heated to 2050 degrees Fahrenheit, plus or minus 25 degrees Fahrenheit, by induction heating, causing the copper infiltrant to melt and infiltrate the heated powder mixture in the cavity through capillary action. Once infiltrated, it is slowly cooled to room temperature. After cooling, the parts are machined to specific dimensions by grinding.
Cemented tungsten carbide is one example of a hard composite material fabricated by mixing together a powder formed of particles of a carbide of one of the group IVB, VB, or VIB metals, with a metal binder in powdered form, pressing the mixture into a desired shape to form a “green part,” and then sintering the green part to cause the binder to melt and thereby form an agglomeration of carbide particles bonded together by the metal binder phase. The binder material is typically comprised predominantly of cobalt, nickel, or iron, and alloys of them. The most common example of a cemented metal carbide composite used in downhole applications is tungsten carbide (WC) with a cobalt binder.
Microwave sintering of metal carbides with a metal alloy as a catalyst or binder phase material is described in several patents, including U.S. Pat. Nos. 6,004,505, 6,512,216, 6,610,241, 6,805,835, all of which are incorporated herein by reference. In a microwave sintering process, loose grains of metal carbide, which constitute a metal carbide powder, and a metal binder powder are combined to form an homogenous mixture, which is then shaped or formed into a “green” part that has very near the dimensions and shape of a desired cemented metal carbide part. The green part is formed, for example, by compacting the carbide and binder powders into a mold by cold pressing. It may also be precast with a sacrificial wax if necessary. One example of a metal carbide is tungsten carbide. The metal binder that is typically used is a metal alloy containing about 80 to 100 percent cobalt. Additional materials can also be added to the mixture. The green part is then sintered using microwave radiation to heat the part to a point that is below the melting temperature of the metal carbide, but high enough to cause the metal binder to melt throughout the matrix of metal carbide grains, resulting in the particles of carbide fusing or adhering to one another to thereby form a single, solid mass. Microwave heating shortens sintering times. Shorter sintering times result in less chemical and phase change in the metal binder, which is typically cobalt or an alloy containing cobalt. More even heating is also possible, which results in more uniform shrinkage of the part and more uniform distribution of the binder during cooling. Shorter sintering times also result in smaller changes in the size of the grains. Smaller changes in the grain size result in more predictable and consistent carbide grain structures. Microwave sintering also allows for uniform cooling after sintering, which allows for better management of stresses within the part and better phase control of the metal binder. A microwave sintered metal carbide part typically possesses higher modulus of elasticity, yield strength, and impact strength and greater thermal and electric conductivity as compared to a part having the same starting materials sintered using conventional HP/HT and HIP methods.